1. Field of the Invention
This invention relates to an improved method for the manufacture of acetic acid.
2. The Related Art
An important process for the production of acetic acid is the carbonylation of an alkyl alcohol, especially methanol, and reactive derivatives thereof, with carbon monoxide in a liquid reaction medium. Such carbonylation reactions are generally carried out in the presence of a catalyst, e.g., a Group VIII metal catalyst such as rhodium and iridium, a halogen containing catalyst promoter, e.g., methyl iodide, and water. U.S. Pat. No. 3,769,329 discloses the use of a rhodium-based carbonylation catalyst dissolved, or otherwise dispersed, in a liquid reaction medium or supported on an inert solid, along with a halogen-containing catalyst promoter as exemplified by methyl iodide. However, it is understood that various catalyst systems, particularly those incorporating Group VIII metals, may be used for the production of acetic acid through the carbonylation of methanol. Generally, the carbonylation reaction is conducted with the catalyst being dissolved in a liquid reaction medium through which carbon monoxide gas is continuously bubbled. U.S. Pat. No. 3,769,329 discloses that water may be added to the reaction mixture to exert a beneficial effect upon the reaction rate, and water concentrations between about 14 weight percent (wt. %)–15 wt. % are typically used. This is sometimes referred to as the “high water” carbonylation process.
An alternative to the “high water” carbonylation process is the “low water” carbonylation process, as described in U.S. Pat. Nos. 5,001,259, 5,026,908, and 5,144,068. Water concentrations below 14 wt. % can be used in the “low water” carbonylation process. Employing a low water concentration simplifies downstream processing of the desired carboxylic acid to its glacial form. The more water there is in a reaction stream, the greater the operating costs to remove water from the product acetic acid and the greater the capital investment in product recovery and purification equipment. The efficiencies achieved when operating at very low water concentrations makes it attractive to operate at the lowest water concentration possible. However, when reducing the reactor water to minimize operating and fixed costs, it is more difficult to maintain acceptably high rates of acetic acid production with good catalysts stability since the rate of the reaction decreases, as the reactor water is decreased as explained in U.S. Pat. No. 5,026,908.
One of the problems associated with low water production is that catalyst systems, especially rhodium-based catalysts, tend to precipitate out of the reaction mixture when the concentration of water is decreased, especially at concentrations lower than 14 wt. %. Significant catalyst precipitation, of course, can lead to reduced reaction rates, interrupted unit operation, and complete shutdowns. It is known that catalyst stability problems may be minimized by the use of a catalyst stabilizer such as a soluble metal iodide or quaternary iodide salt. As discussed in U.S. Pat. No. 5,218,143, especially suitable salts are the alkali metal iodides such as lithium iodide since these are the most soluble and thermally stable in the reaction medium. EP-A-0161874 describes a reaction system in which an alcohol, as exemplified by methanol, is carbonylated to a carboxylic acid derivative such as acetic acid while using a liquid reaction medium having a low water content. The disclosure describes that this is achieved by the use of defined concentrations of an iodide salt, alkyl iodide and corresponding alkyl ester in the liquid reaction medium to maintain rhodium catalyst stability and system productivity.
An additional problem associated with carbonylation reactions at lower water concentrations is that, even when catalyst systems are stabilized, production rates are adversely reduced. For example U.S. Pat. No. 5,760,279 discloses than when operating under low water conditions, the realized reaction rate may be less than half of what would normally be expected under a given set of conditions.
Various techniques for increasing the production rate under low water carbonylation reaction conditions have been proposed. Production rates are typically defined in terms of space-time yield (STY) which is expressed in gram-moles of acetic acid produced per hour per liter of reaction medium (g-moles/l/hr) contained in the carbonylation reactor. The volume of the reaction medium is being determined at ambient temperature in the unaerated state. U.S. Pat. No. 5,218,143 discloses that production levels may be enhanced at low water levels if the reactor is operated with optimized concentrations of methyl acetate in the reaction mixture. EP-0-250189 proposes to add hydrogen gas in the carbon monoxide feed to the reaction mixture to enhance the production rate. U.S. Pat. No. 5,939,585 discloses the use of ruthenium or osmium as catalyst promoters to enhance production rates. The disclosure of this patent indicates the use of such promoters may result in STY's of up to approximately 11 g-mol/l/hr under low water conditions at concentrations of less than 1.0 wt. % water. U.S. Pat. No. 5,218,143 discloses the use of Group VIB metal catalyst co-stabilizers for increasing STY's under low water conditions to as high as 9.2 g-mol/l/hr at a water concentration of 2.0 wt. %. U.S. Pat. No. 5,760,279 indicates that the incorporation of a manganese stabilizer in conjunction with a rhodium catalyst may increase STY's up to approximately 8 g-mol/l/hr at a water concentration of 4.5 wt. %. U.S. Pat. No. 5,488,153 and GB 2,336,154 A propose the use of bidentate phosphorus-sulfur ligands coordinated to rhodium catalysts for increasing reaction rates under low water conditions. The examples of U.S. Pat. No. 5,488,153 disclose the achievement of production rates up to an STY of 19.6 g-mol/l/hr. GB 2,336,154 A discloses reaction rates as high as 21.9 g-mol/l/hr. These reactions disclosed in these references take place under high water conditions.
While some of the above references refer to rhodium catalyst concentrations as high as 5000 ppm, the examples in these references generally disclose rhodium catalyst concentrations of about 1000 ppm or less.
U.S. Pat. No. 5,144,068 discloses that, at low water concentrations, there is a synergy between the methyl acetate and iodide salt stabilizer in the carbonylation reactor to enhance methanol carbonylation. It also discloses that an advantage of operating the reactor at high methyl acetate concentrations is a reduction in the formation of undesirable reaction products. In particular, propionic acid is reduced by an order of magnitude. Carbon dioxide and hydrogen, which are formed by the water gas shift reaction, are also reduced.
Because the carbonylation rate of reaction is strongly dependent on water concentrations, it is important to maintain water levels in the reaction mixture during the production of acetic acid within controlled ranges to maintain high reaction rates. Hjortkjaer and Jensen [Ind. Eng. Chem., Prod. Res. Dev. 16, 281–285 (1977)] discloses the strong dependence of the rate of reaction on water levels by demonstrating that the reaction rate increases as water concentration is increased up to 14 wt. %. The control of water in the reaction mixture can be affected, at least in part, by two key reactions in the reaction mixture. The first reaction produces water through methanation in accordance with the following formula:CH3OH+H2→CH4+H2O
The second reaction which consumes water is known as the aforementioned water gas shift reaction shown by the following formula:CO+H2O→CO2+H2
To effectively control water in the reaction medium, it is important to know which reaction predominates in order to define a water supply or water removal operation from the reaction section to maintain an accurate water balance within the reaction section to minimize changes in reaction carbonylation rates as a result of changes in the reactor water concentration.
U.S. Pat. No. 5,831,120 discloses that in iridium-catalysed carbonylation reactions, the generation rate of water by the methanation reaction is relatively high and can be greater than the rate of consumption of water by the water gas shift reaction. In this situation there is a need to remove excess water generated by the imbalance. In contrast, U.S. Pat. No. 5,831,120 also discloses that in rhodium-catalysed carbonylation reactions, the methanation reaction is relatively slow compared to the rate of the water gas shift reaction so water is consumed in the reaction system. It is typically necessary to provide water to the rhodium-catalysed system to maintain a steady-state concentration of water in the reaction mixture.
Various means have been proposed for removing excess water from crude product streams produced in carbonylation reaction systems. U.S. Pat. Nos. 3,769,177 and 3,791,935 disclose the removal of water from reaction systems through a series of distillations. U.S. Pat. No. 4,008,131 discloses a modification of such systems by using a sidestream for removal of water from a distillation column. The purported advantage of such a system is to minimize the removal of valuable methyl iodide with the water when it is removed from the overhead from a distillation column. The process systems disclosed in these patents are directed to means for removing water from crude product streams in the post reaction section portions of the process systems. Therefore, the disclosed systems do not address controlling water in the reaction section of carbonylation process systems.
U.S. Pat. No. 5,831,120 discloses the removal of excess water in an iridium-catalysed system by a combination of removing and disposing of water from the overhead of a light ends distillation column and replacing a portion of the methanol feed into the reaction mixture with a component selected from the group of methyl acetate, dimethyl ether, acetic anhydride and mixtures thereof. The patent discloses that dimethyl ether and methyl acetate are carbonylated to produce acetic acid with a net consumption of water and acetic anhydride removes water from the reaction mixture by reaction to produce acetic acid. In this process, water is thought to be consumed in accordance with the following formulas: 
U.S. Pat. No. 5,001,259, and related U.S. Pat. Nos. 5,026,908, and 5,144,068 disclose rhodium catalysed low water carbonylation processes in which high methanol carbonylation rates are achieved while the reaction section water concentrations are maintained at very low levels from a finite (≦0.1 wt. %) water concentration to high water concentrations. These patents disclose that the reaction medium concentration is maintained by controlling the flow of carbon monoxide, water, methanol, and methyl iodide. The highest acetic acid production reaction rate disclosed in these patents is an STY of approximately 32 at a water concentration of 2 wt. %. However, at water concentrations of less than 2 wt. %, the highest acetic acid STY disclosed is approximately 12. FIG. 10 of these patents demonstrates the difficulty of maintaining favorable reaction rates at water concentrations below 2.0 wt. %. As seen in FIG. 10, the reaction rate drops precipitously as the water concentration goes below 2.0 wt. %.
In summary, the state of the art in carbonylation technology still lacks a method for maintaining a highly stable catalyst system, in controllable low water conditions, useful for achieving reaction rate STY's of 15 g-mol/l/hr and higher at water concentrations of less than 2 wt. %.